Normally, paper is made up of lignocellulose-containing fibers bound to each other by hydrogen bonds. To give the finished paper certain desirable properties, the paper often contains specific paper chemicals, so-called function chemicals, for instance sizing agents, dry strength agents and wet strength agents. In the production of paper, also process chemicals are often used to improve production efficiency. Examples of such chemicals are retention agents, dewatering agents, defoamers and slime controlling agents.
Paper is primarily produced according to the wet process, in which a suspension of lignocellulose-containing fibers, water and, usually, one or more paper chemicals are dewatered on a water-permeable cloth (wire), thereby forming a fiber web or sheet which is pressed and dried into finished paper.
Many types of paper come into contact with liquids, primarily aqueous solutions or water vapour. Since the fibers have a strong attraction for water, i.e. they are hydrophilic, they will absorb water, which weakens the paper. This effect can be counteracted by coating the fibers with a water-repellent, i.e. hydrophobic, substance which reduces the possibility of penetration of liquid into the ready-dried web or sheet. For this purpose, use has previously been made of e.g. tall oil rosin from the sulphate process, paraffin wax dispersions, sodium stearate and cellulose-reactive sizing agents. In the production of paper, the hydrophobic substances are usually introduced by addition to the suspension of fibers and water (the stock), so-called stock sizing. Examples of paper which is sized includes liquid carton board, fine paper and kraft liner.
Japanese patent specification JP 62299/80 discloses paper containing zeolites. According to the Japanese specification, the paper contains a hydrophilic zeolite, mordenite, which increases the water-absorbing capacity of the paper, i.e. the opposite to what is desirable with a sizing agent.
Swiss patent specification CH 678636 discloses sizing of paper and paperboard by adding a sizing agent comprising a natural or synthetic resin in combination with an inorganic matrix containing aluminium and silicon. The insoluble inorganic matrix is suitably a natural or synthetic zeolite. The zeolites mentioned in this specification are either entirely hydrophilic or can be hydrophilic or hydrophobic depending on the pretreatment they have been subjected to. There is no information that the zeolites should be strongly hydrophobic since the aim of the zeolites mentioned in CH 678636 is to improve the retention of the sizing agent and not to act as sizing agents themselves.
Further, it is known to use natural zeolites as fillers in papermaking. Such natural zeolites are hydrophilic by being rich in aluminium as well as having a residual butanol content of 1.0, or very close to 1.0, when the hydrophobicity is determined in accordance with the so-called Residual Butanol Test.